Vacuum forming is typically used for forming thick (50-120 mils) heat-deformable articles such as plastic refrigerator door liners. Additionally, vacuum forming is a common method of encapsulating one or more articles within a plastic film material, usually for the purpose of shipping the articles as a package. Techniques for wrapping articles by this method are well known and are distinct from heat-shrink wrapping of articles. In a vacuum forming operation plastic film material, having a thickness in the range of 10-100 miles, is softened by heat and formed to the desired shape by vacuum, whereas in shrink wrapping the film is heated to cause it to shrink about an article. Although the plastic material used in a vacuum forming operation may shrink slightly during the process, perhaps in the order of a few percent of its original size, it is insignificant compared to the shrinkage of film in a shrink wrap operation where the film may readily shrink 75% in the main direction and 30% in the cross direction.
There are reasons other than for ease of handling for wrapping articles with plastic film, one being to prevent fibrous articles from releasing fibers to the environment. An example is in the manufacture of automobile hoodliners formed of fiber glass. Such hoodliners are shaped by molding and trimming a bonded fiber glass mat to the contour of an automobile hood. Workers in the assembly line who are located beneath installed hoodliners are at times exposed to fibers broken and released from such fiber glass hoodliners, particularly if the hoodliners are subjected to vibration or shock. As a way of eliminating this, it has been suggested to encapsulate the fiber glass hoodliners in a thin film. The contoured shape of hoodliners, however, raises problems.
To encase a hoodliner in a plastic film by the shrink-wrap method would be too expensive due to the type of plastic film required compared to the plastic material used in vacuum forming operations and, in encasing contoured articles, utilization of the shrink-wrap material and method causes the film, quite often, to bridge the contoured surfaces of the encased article and not conform exactly to the surface as is required for maintaining the desired shape of the hoodliner.
To overcome the bridging tendency, a vacuum forming process could be used wherein the plastic film would be supported on a frame and heated to its softening temperature and then lowered into contact with an upper face of a contoured article. Holes in the mold would permit a vacuum to be applied, through the opposite side of the permeable contoured article, or the uncoated side, to draw a vacuum in the volume between the mold and the plastic film, drawing the film down onto the top surface of the article and conforming it to the exact contour thereof. This process would be effective to coat one face of the article; however, to coat the other face would require inverting the article in another mold, supporting it with a contoured surface conforming to that surface and again heating a film of plastic and applying it to the then top surface. However, to evacuate the volume between the last applied film and the mold would require piercing the previously applied film to be able to apply an appropriate vacuum to draw the film onto the article. As is evident, such a process requires a multiple step operation utilizing contoured molds and further requires the use of a relatively tough film capable of withstanding the stresses to which it is subjected during such handling and encapsulating process. Further, such method would typically leave the marginal edges of the hoodliner uncoated, thereby yielding an article that still exposed workers, through subsequent handling, to edge fibers from the hoodliner.
The aforementioned commonly assigned patent application discloses a method of encapsulating a molded fiber glass hoodliner using heat and vacuum to conform the plastic film onto the hoodliner; however, such method is not amenable to automated in-line production techniques and still requires undesirable manual operations.